Refactor methods related to localized strings in Date

[Vellumic]

Refactoring of the logic added by 26271b7. This PR is a prerequisite for #5199

It changes the following:

• Introduced a shared implementation for toLocaleString, toLocaleDateString, and toLocaleTimeString
• Removed format_date_time_locale as unnecessary
• Some related minor improvements

Testing:

• cargo test -p boa_engine --lib -- date::tests::date_proto_to_locale_string_intl (skipped)
• cargo test -p boa_engine --features intl_bundled --lib -- date::tests::date_proto_to_locale_string_intl (passed)

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,125	53,125	0
Passed	51,072	51,072	0
Ignored	1,482	1,482	0
Failed	571	571	0
Panics	0	0	0
Conformance	96.14%	96.14%	0.00%

Tested main commit: 7ce9cae84ac9bf54fe528bb05b997ac24af808a4
Tested PR commit: a8b821675d338bb47f97a75b5cd24738141fc88a
Compare commits: 7ce9cae...a8b8216

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.34%. Comparing base (6ddc2b4) to head (a8b8216).
⚠️ Report is 982 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/date/mod.rs	93.33%	1 Missing ⚠️
...e/engine/src/builtins/intl/date_time_format/mod.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5411       +/-   ##
===========================================
+ Coverage   47.24%   60.34%   +13.10%     
===========================================
  Files         476      567       +91     
  Lines       46892    63112    +16220     
===========================================
+ Hits        22154    38086    +15932     
- Misses      24738    25026      +288     

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